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But changes in diet may not help protect those who are already experiencing problems with memory, the researchers said.

The study measured levels of biomarkers associated with Alzheimer's disease, including certain proteins found in spinal fluid.

Over the course of the four-week study, healthy participants who ate an unhealthy diet — one high in saturated fats and foods that quickly increase blood sugar — saw levels of these biomarkers change in a way that could indicate detriments to brain health, the researchers said.

In contrast, those that ate a healthy diet saw the opposite effect on these biomarkers.

"Diet may be a powerful environmental factor that modulates Alzheimer disease risk," the researchers said.

Participants who already had mild memory problems saw mixed results: a healthy diet decreased some, but not all, of the biomarkers associated with Alzheimer's disease. This suggests dietary interventions are less effective once cognitive impairment has begun, the researchers said.

The results are preliminary and more work is needed to confirm the findings. So far, researchers do not have enough evidence to say that changes in diet — or any other behavioral factor, for that matter — can prevent Alzheimer's disease, according to a report released last month. Additional work is also needed to pin down specific biomarkers for Alzheimer's.

The new study will be published in the June issue of the journal Archives of Neurology.

The new study involved 20 older adults who were healthy and 29 who had amnestic mild cognitive impairment (aMCI), meaning they experienced some memory problems. The participants were randomly assigned to eat either the healthy or unhealthy diet for four weeks. The researchers studied participants' performance on memory tests as well as their levels of biomarkers, such as insulin, cholesterol, blood glucose levels, blood lipid levels and cerebrospinal fluid proteins.